The invention relates to the use of thermoplastic molding compositions comprising    A) a polyester,    B) from 20 to 200 mmol/kg of polyester A) of at least one compound of the general formula (I)
                where        respectively independently at any position        -A1- is —NR—, —O—, —S—, —CH=A4- where R is H or C1-6-alkyl, A4 is N or CH        A2 is COOX or OX, where X is Li, Na, K, Rb, Cs, Mg/2, Ca/2, Sr/2, Ba/2, Al/3        A3 is C1-6-alkyl, C6-12-aryl, C7-13-alkaryl, C7-13-aralkyl, O—C1-6-alkyl, O—C6-12-aryl, O—C7-13-alkaryl, O—C7-13-aralkyl, COOX′, OX′, SX′, SO3X′, where X′ is H or X, S—C1-6-alkyl, S—C6-12-aryl, NR2, halogen, NO2,        n is an integer from 1 to 4, and        m is an integer from 0 to 4−n, where m=1, if A3=NO2,        with the proviso that the number of mmol is based on the group(s) COOX and OX and SX′ where X′═X, to the extent that these are present in the compound of the general formula (I), and also moreover            C) from 0 to 230% by weight of further added substances, based on the weight of component A), for producing laser-transparent moldings of any type.
The invention further relates to the use of the laser-transparent moldings for producing moldings by means of laser transmission welding processes, to processes for producing moldings of this type, and also to the use of these in various application sectors.
Components B) of this type have been described by way of example in Polymer Engineering and Science 1990, BO (5), pp. 270 ff, and 1995, 35 (17), pp. 1407 ff, Journal of Appl. Pol. Sci. 2004, 93, pp. 590 ff, and also U.S. Pat. No. 4,393,178, and EP-A-0 251 732, as nucleators for compounded PET materials. The optical properties of the compounded materials were not studied.
There are various existing processes for the welding of plastics moldings (Kunststoffe 87, (1997), 11, 1632-1640). Precondition for a stable weld in the widely used heated-tool welding process and vibration welding process (e.g. for motor-vehicle intake manifolds) is adequate softening of the adherends in the contact zone prior to the actual connection step.
Laser transmission welding, in particular using diode lasers, has been increasingly widely used recently as a method providing an alternative to vibration welding and heated-tool welding.
The technical literature describes fundamental principles of laser transmission welding (Kunststoffe 87, (1997) 3, 348-350; Kunststoffe 88, (1998), 2, 210-212; Kunststoffe 87 (1997) 11, 1632-1640; Plastverarbeiter 50 (1999) 4, 18-19; Plastverarbeiter 46 (1995) 9, 42-46).
Precondition for the use of laser transmission welding is that the radiation emitted by the laser first penetrates a molding which has adequate transparency for laser light of the wavelength used and which in this application is hereinafter termed laser-transparent molding, and then is absorbed in a thin layer by a second molding which is in contact with the laser-transparent molding and hereinafter is called laser-absorbent molding. Within the thin layer which absorbs the laser light, the laser energy is converted into heat, and this leads to melting within the contact zone and finally to a weld which bonds the laser-transparent molding to the laser-absorbent molding.
Laser transmission welding usually uses lasers in the wavelength range from 600 to 1200 nm. Within the wavelength range of the lasers used for the thermoplastics welding, the usual lasers are Nd:YAG lasers (1064 nm) or high-power diode lasers (from 800 to 1000 nm). When the terms laser-transparent and laser-absorbent are used hereinafter, they always relate to the abovementioned wavelength range.
The laser-transparent molding, unlike the laser-absorbent molding, requires high laser transparency within the preferred wavelength range, so that the laser beam can penetrate as far as the area of the weld, with the energy level required. An example of a method used to measure capability to transmit IR laser light uses a spectrophotometer and an integrating photometer sphere. This measurement arrangement also detects the diffuse fraction of the transmitted radiation. The measurement is made not only at a single wavelength but within a spectral range which comprises all of the laser wavelengths currently used for the welding procedure.
Users presently have access to a number of laser-welding-process variants based on the transmission principle. By way of example, contour welding is a sequential welding process in which either the laser beam is conducted along a freely programmable weld contour or the component is moved relatively to the immovable laser. In the simultaneous welding process, the linear radiation emitted from individual high-power diodes is arranged along the contour of the weld. The melting and welding of the entire contour therefore take place simultaneously. The quasi-simultaneous welding process is a combination of contour welding and simultaneous welding. Galvanometric mirrors (scanners) are used to conduct the laser beam at very high velocity at 10 m/s or more along the contour of the weld. The high traverse rate provides progressive heating and melting of the region of the joint. In comparison with the simultaneous welding process, there is high flexibility for alterations in the contour of the weld. Mask welding is a process in which a linear laser beam is moved transversely across the adherends. A mask is used for controlled screening of the radiation, and this impacts the area to be joined only where welding is intended. The process can produce very precisely positioned welds. These processes are known to the person skilled in the art and are described by way of example in “Handbuch Kunststoff-Verbindungstechnik” [Handbook of plastics bonding technology] (G. W. Ehrenstein, Hanser, ISBN 3-446-22668-0) and/or DVS-Richtlinie 2243 “Laserstrahlschweiβen thermoplastischer Kunststoffe” [German Welding Society Guideline 2243 “Laser welding of thermoplastics”].
Irrespective of the process variant used, the laser welding process is highly dependent on the properties of the materials of the two adherends. The degree of laser transparency (LT) of the transparent component has a direct effect on the speed of the process, through the amount of energy that can be introduced per unit of time. The inherent microstructure, mostly in the form of spherulites, of semicrystalline thermoplastics generally gives them relatively low laser transparency. These spherulites scatter the incident laser light to a greater extent than the internal structure of a purely amorphous thermoplastic: back-scattering leads to reduced total amount of transmitted energy, and diffuse (lateral) scattering often leads to broadening of the laser beam and therefore to impaired weld precision. These phenomena are particularly evident in polybutylene terephthalate (PBT), which in comparison with other thermoplastics that crystallize well, such as PA, exhibits particularly low laser transparency and a high level of beam expansion. PBT therefore continues to be comparatively little used as material for laser-welded components, although other aspects of its property profile (e.g. good dimensional stability and low water absorption) make it very attractive for applications of this type. Although semicrystalline morphology is generally unhelpful for high laser transparency, it provides advantages in terms of other properties. By way of example, semicrystalline materials continue to have mechanical strength above the glass transition point and generally have better chemicals resistance than amorphous materials. Materials that crystallize rapidly also provide processing advantages, in particular quick demoldability and therefore short cycle times. It is therefore desirable to combine semicrystallinity with rapid crystallization and high laser transparency.
There are various known approaches to laser-transparency increase in polyesters, in particular PBT. In principle, these can be divided into blends/mixtures and refractive-index matching.
The approach using blends/mixtures is based on “dilution” of the low-laser-transparency PBT by using a high-laser-transparency partner in the blend/mixture. Examples of this are found in the following specifications: JP2004/315805A1 (PBT+PC/PET/SA+filler+elastomer), DE-A1-10330722 (generalized blend of a semicrystalline thermoplastic with an amorphous thermoplastic in order to increase LT; spec. PBT+PET/PC+glass fiber), JP2008/106217A (PBT+copolymer with 1,4-cyclohexanedimethanol; LT of 16% increased to 28%). A disadvantage here is that the resultant polymer blends inevitably have properties markedly different from those of products based predominantly on PBT as matrix.
The refractive-index-matching approach is based on the different refractive indices of amorphous and crystalline PBT, and also of the fillers. By way of example, comonomers have been used here: JP2008/163167 (copolymer of PBT and siloxane), JP2007/186584 (PBT+bisphenol A diglycidyl ether) and JP2005/133087 (PBT+PC+elastomer+high-refractive-index silicone oil) may be mentioned as examples. Although this leads to an increase in laser transparency, this is achieved with loss of mechanical properties. The refractive-index difference between filler and matrix can also be reduced, see JP2009/019134 (epoxy resin coated onto glass fibers in order to provide matching at the optical interface between fiber and matrix), or JP2007/169358 (PBT with high-refractive-index glass fiber). Starting materials of this type are, however, disadvantageous because of their high costs and/or the additional stages that they require within the production process.
The effects achieved in relation to laser-transparency increase are also overall relatively minor and therefore not entirely satisfactory.